1. Field of the Invention
The present invention relates to oil pumps and particularly to an oil pump for an automatic transmission system employed in a vehicle such as automobile.
2. Description of the Background Art
An oil pump for an automatic transmission used in a vehicle like automobile is constituted of a rotor that revolves and an oil pump housing that encases the rotor. The rotor and oil pump housing are generally formed of iron (cast iron).
In recent years, reduction in weight of automobiles as well as oil pumps for automatic transmissions has been required for improvement of fuel economy. Then, use of aluminum alloy has been considered as a material constituting the oil pumps. However, the rotor, as a component of the oil pump, is made of an iron-based material because of the requirement of a high wear resistance of the rotor and a low weight ratio of the rotor itself to the entire oil pump, and the like. On the other hand, the oil pump housing is effectively constituted of aluminum alloy for reducing the weight since the weight of the oil pump housing accounts for most of the total weight of the oil pump.
If the rotor made of the iron-based material and the housing made of aluminum alloy are combined, a problem arises that a part of the oil pump housing, along which the rotor slides while contact is kept therebetween, is likely to wear because of an inferior wear resistance of the aluminum alloy.
An invention for improving the wear resistance of the oil pump housing is disclosed, for example, in Japanese Utility Model Publication No. 3-15832. According to this invention, the wear resistance is enhanced by dispersing ceramic fibers in a part of the oil pump housing, along which the rotor slides while contacting therewith, and thus producing a composite material.
This invention has a problem in terms of handling that the formability of the ceramic fibers is poor because the ceramic fibers are chopped fibers and accordingly the shape is likely to be lost. When ceramic fibers are impregnated with aluminum alloy to produce a composite, the impregnation requires a significantly high pressure. Consequently, any special equipment is necessary which increases equipment cost. Further, the shape of a mold is limited and accordingly the degree of freedom of pump design is restricted. There is a further problem in the actual manufacture that machinability in a cutting process after the composite is produced is poor.
The present invention is made to solve the problems mentioned above. One object of the invention is to provide a lightweight oil pump that is superior in wear resistance and productivity.
An oil pump according to the present invention includes an oil pump housing formed of aluminum alloy and a pump element sliding along the oil pump housing while contacting therewith to suck and discharge oil. A porous metallic body having a foam structure is embedded in a part of the oil pump housing that contacts the pump element, and pores of the porous metallic body are impregnated with the aluminum alloy constituting the oil pump housing.
The oil pump with such a structure has the porous metallic body embedded in the part contacting the pump element, and thus the wear resistance of the part contacting the pump element is improved. The oil pump housing is made of the aluminum alloy which is light, and thus the oil pump can be reduced in weight. Further, the porous metallic body is easily processed, cut, for example, and has a sufficient stiffness as a structure owing to the metallic properties and thus the porous metallic body is easily processed and formed into any complex shape and maintained as it is, providing a superior productivity.
Impregnation with the aluminum alloy is easily accomplished because of the form structure and accordingly manufacture requires no special equipment. Consequently, a lower equipment cost and fewer limitations of the mold shape are achieved which enhances the degree of freedom of pump design. Compared with the ceramic fibers, the porous body impregnated with aluminum alloy has an improved machinability and thus an oil pump superior in wear resistance and productivity can be provided.
Preferably, the pump element has a rotor that revolves and a porous metallic body is embedded in a part that contacts a side surface of the rotor.
Still preferably, the pump element has a rotor that revolves and a porous metallic body is embedded in a part that contacts the peripheral surface of the rotor.
Still preferably, the average pore diameter of the porous metallic body is at least 0.1 mm and at most 3.0 mm. The porous metallic body according to the present invention has the structure as shown in FIG. 4. The average pore diameter of the porous metallic body is measured in the following way. First, a picture is taken of an arbitrarily selected cross section of the body, the picture corresponding to a rectangular photographic field. Then, respective lengths of pores crossed by two diagonal lines in the rectangular field are measured. Finally, the sum of the pore lengths is divided by the total number of those pores and accordingly the average pore diameter is determined. It is noted that the pore diameter of the porous metallic body herein refers to the general term used in the art that represents an average diameter of pores of a base material such as urethan foam.
The optimized average pore diameter allows easier impregnation with the aluminum alloy and improves the wear resistance. If the average pore diameter is less than 0.1 mm, the smaller pores deter impregnation with the aluminum alloy. If the average pore diameter exceeds 3.0 mm, the area of the exposed skeleton of the porous metallic body per unit area decreases, which lowers the effect of enhancing wear resistance.
Still preferably, the volume fraction of the porous metallic body is at least 2% and at most 30%. Here, the volume fraction is calculated from (apparent density: density calculated from the outer diameter and weight)/(density of the metallic material constituting the porous metallic body: density of metal)xc3x97100%. It is noted that the apparent density is identical in meaning to bulk density, and the density of the metallic material constituting the porous metallic body is identical in meaning to the true density of the metallic material constituting the porous metallic body. The volume fraction represents the amount of metal contained in a certain volume. For example, the volume fraction of 30% means that the metal accounts for 30% of that certain volume and vacancies where no metal exists account for 70% thereof.
This optimized volume fraction can improve the wear resistance and reduce the weight. If the volume fraction is less than 2%, the area of exposed porous metallic body is smaller, which lowers the effect of enhancing the wear resistance. If the volume fraction is more than 30%, the weight of the porous metallic body increases and thus the advantage of reducing the weight cannot be achieved while the wear resistance remains the same.
Still preferably, the porous metallic body contains at least one selected from the group consisting of iron (Fe), nickel (Ni) and chrome (Cr). These metals all have a higher hardness than that of aluminum alloy and thus the wear resistance is improved. For the purpose of cutting the manufacturing cost, a material containing a relatively great amount of iron is preferable. Further, a material containing iron and chrome is more preferable in order to enhance the hardness.
Still preferably, the porous metallic body is formed by sintering. Specifically, metallic powder is attached to urethan foam and the metallic powder is sintered to produce an alloy simultaneously with burning down of the urethan foam. In the process of sintering, the powder shrinks due to the sintering so that the metallic skeleton constituting the porous metallic body becomes solid. Then, all pores are impregnated with aluminum alloy and an enhanced wear resistance is exhibited. Alternatively, the porous metallic body can be produced by forming a metallic layer on a surface of urethan foam through plating. If the plating is employed for production, the urethan foam as a base material is burned down after a metallic skeleton is formed. Because of this, the skeleton structure of the porous metallic body is hollow. In this case, any defective portion could be generated that cannot be impregnated with molten aluminum alloy.
Still preferably, metal constituting the porous metallic body has a Vickers hardness of at least 100 and at most 1000. The optimized Vickers hardness enhances the wear resistance as well as workability. In addition, any loss or damage due to friction can be eliminated. If the Vickers hardness of the metal constituting the porous metallic body is less than 100, which is almost equal to the hardness of aluminum alloy, the effect of enhancing the wear resistance by producing a composite cannot be achieved. If the Vickers hardness of the metal constituting the porous metallic body exceeds 1000, the porous metallic body becomes brittle resulting in a problem in terms of workability that breakage occurs when a preform is formed for producing a composite. Further, friction in operation of the oil pump causes loss and damage, resulting in deterioration of the wear resistance. If the metal is too hard, the metal delivers a severer attack on the subject material (rotor) to increase damage to the rotor and accordingly any defect occurs.
Still preferably, metal constituting the porous metallic body has a Vickers hardness of at least 120 and at most 300.
Still preferably, the oil pump housing is formed through any of squeeze cast process, die casting process and low pressure die casting process to make the composite of the porous metal and the aluminum alloy through casting. In particular, the low pressure die casting can be advantageous in terms of low equipment cost, and reduced limitations of the shape of a mold and thus an enhanced degree of freedom of pump design.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.